It has heretofore been suggested that substantial savings in energy can be achieved when there is no load on a pump by utilizing a valve which closes the pump inlet, except for a small opening to admit a prescribed small volume of fluid to lubricate and cool the pump. Such a valve is commonly known as a dry valve or a cruise valve and is described, for example, in U.S. Pat. Nos. 2,118,180 and 3,935,917.
In such a hydraulic circuit, if the valve at the pump inlet is closed when the hydraulic system is loaded, the resulting sudden decompression of the pressurized volume and the ensuring cavitation will cause damage to the pump. In addition, the noise attributed to cavitation is extremely loud and usually intolerable. In addition, in such a hydraulic system, it is necessary that when the valve is closed, the amount of flow permitted to enter the pump and its pressure be controlled to certain minimum limits so as to reduce the conditions for cavitation while the pump is operating. If this amount of lubricating/cooling flow and its pressure exceed a certain minimum, the pump will operate with noticeable cavitation.
Accordingly, among the objectives of this invention are to provide a hydraulic system wherein excessive cavitation due to unloading the pump when the hydraulic system is loaded is obviated; wherever the amount of hydraulic fluid which is permitted to pass through the pump when the dry valve is closed is controlled to reduce cavitation and to prevent high levels of noise; and wherein the pressure of the fluid which is circulated through the pump when the valve is closed is controlled to assure quiet operation.
In accordance with the invention, a hydraulic system comprises a pump, a first valve in the inlet of the pump, a second valve which activates or deactivates a third valve which permits the loading and unloading of the pump discharge. A motor is provided to operate the first valve for opening or closing the pump inlet. This motor also actuates the second valve in concert with the operation of the first valve to open or close the vent portion to the third valve which opens or closes the passage from the pump discharge to tank, away from the pump inlet. This permits unloading the pump discharge during the restriction of the pump inlet and for allowing the loading of the pump discharge after the opening of the pump inlet. With the pump inlet restricted, the system provides a low restrictive passage for circulating the prescribed small volume of flow used to lubricate and cool the pump.